The present invention relates in general to air registers for furnaces and boilers and, in particular, to air registers designed to admit and to regulate air necessary to support combustion in industrial furnaces and boilers. Specifically, the invention relates to air registers used with coal, oil or gas fired steam generating boilers.
Air registers per se, are well known by those skilled in the art and air registers designed to regulate more than one source of combustion air are also well known. Thus, the prior art is replete with disclosures of air registers referring to so-called primary, secondary and even tertiary combustion air and various means to deliver this air to a boiler. Although each inventor of air registers defines and refers to combustion air in accordance with his own background and concepts, combustion air usually falls into two broad categories. Primary air is generally understood to refer to air which is used as a carrier medium to transport the fuel to the furnace. Powdered coal is a good example of a fuel which is transported in a stream of air through a separate fuel pipe. Since transport air only supplies about 18% of the air needed for full combustion, an additional supply of air must be brought into the boiler from another source and by another means. This "make up" air is sometimes called "secondary air", and is supplied in sufficient volume to provide the additional 82% of required air. However, if a boiler is oil or gas fired, usually little or no supplemental means are required to transport these fuels since thay are generally self-transporting. Thus, substantially 100% of the combustion air must be supplied from some source. Whether air from this source is labeled primary or secondary is a matter of semantics. It is obvious that the source of air supplied through a given register in a powdered coal burning boiler might be referred to as "secondary". On the other hand, a source of air supplied through the same register for use in the same boiler while being operated on oil might be referred to as primary air. In the alternative, the source of air through the air register may be called secondary irrespective of whether the boiler is being coal fired, oil fired or gas fired. One fact is indisputable, the function of the air remains the same irrespective of its label.
For many years a so-called "daisy chain" type damper air register was widely used in the energy generating industry. Examples of this type of register are shown in U.S. Pat. Nos. 2,320,576 and 2,838,103. The dampers for this type of register were positioned immediately adjacent to the throat of the boiler and the complex damper operating mechanism known as a daisy chain has a history of high incidence of failure due to overheating. After a short period of operation the daisy chain linkage would freeze, rendering the dampers inoperable. Thus, operators were afraid to close the dampers during the firing of a boiler because of the high probability that they would freeze in the closed position, thereby rendering the air register inoperable. As a consequence, the daisy chain dampers were usually left wide open at all times, which rendered the register useless as a means of obtaining and controlling efficient fuel combustion.
Recently, a new register has come on the market which is essentially a scroll with a single air entrance remote from the boiler throat and with but one butterfly damper in the air entrance. From a reliability standpoint, this damper would appear to be an improvement over the daisy chain operated type of damper.
Flame shape control is important in boiler operation since the shape of the flame will determine the efficiency of the boiler combustion. In addition to reduced cost of boiler operation, an efficiently operated boiler also reduces the amount of stack flyash; the amount of unburned coal which, when carried to the uppermost reaches of the boiler, cokes and generates a dangerous gas by-product. Nitrous oxide is also a by-product of combustion. Free nitrous oxide escapes up the stack and contributes to air pollution. As presently advised, the U.S. Environmental Protection Agency (EPA) regulations for new boilers restrict the amount of nitrous oxide permitted to escape up the stack to 0.7 pounds per million BTU. Although the EPA has not yet set nitrous oxide emission limitations for old boilers, as will be developed, the present invention enables even old less efficient boilers to operate under the limits of nitrous oxide emission required by the EPA for new boilers.
Since efficiency of combustion is a function of flame shape, if a flame is restricted to a narrow pencil-like shape it is relatively inefficient. As the flame is permitted to open tranversely, its efficiency increases because more room is available within the flame to mix fuel and air. Heretofore, prior art air registers have been the generally accepted means for delivering combustion air to boilers. However, because they do not deliver air efficently, the energy required to deliver air to the boiler results in excessive energy costs. More importantly, prior art registers have not provided the means for controlling the boiler flame in such a manner as to obtain combustion efficiency.